Point contact rectifier of boron phosphide having boron-to-phosphorus atomic ratio of to 100



Sept. 1. 1964 o. a HILL 3,147,413

POINT CONTACT RECTIFIER OF BORON PHOSPHIDE HAVING BORON-TO-PHOSPl-WRUS ATOMIC RATIO OF' 6 1'0 100 Filed Oct. 2'1. 1960 3 Sheets-Sheet. 1

man mg l I III Illlllll I Illll" PRESSURE (MICRONS) I l lllH FIGURE I.

I I Illlll lllllll DALE E. HILL M QM Sgt. 1, 1964 n. E. mu. 3,147,413

Ponrr CONTACT RECTIFIER or aoaon mosnuos wmnc aoaou-m-nmsmows none no or s To 100 3 Shoots-$00! 2 F1106 Oct. 27. 1960 I (millw) DALE E. IILL FIGURE 3.

D. E. HILL 3,147,413 POINT CONTACT RECTIFIER 0F BORON PHOSFHIDE mvmc BORON-TO-PHOSPHORUS ATOMIC RATIO OF 6 T0 100 3 Sheets-Sheet 5 Filed Oct. 27. 1960 d. umPEmuaEuv ozEkiumO NOI .LISOdIIOO DALE mu.

United States Patent Oflice 3,147,413 Patented Sept. 1, 1964 Company a corporation Filed d. 21, 1960, Ser. No. 65,540 18 Claims. or. 311-231 This invention relates to inorganic point contact rectifier devices. More particularly, the invention contemplates the use of new crystalline rhombohedral boron phosphidcs having a boron-to-phosphorus atomic ratio of at least 6 to 1, as the semiconductor component in point contact rectifier devices, the preferred material being the compound B 1. These components can suitably be in the form of discs, wafers, bars, rods, rectangular parallelepipeds, round or any other geometrical shape; however, preferred shapes are thin discs or plates.

It is an object of this invention to provide new and useful point contact rectifier devices which have high thermal stability and high rectification ratios.

This and other objects of the invention will become apparent as the detailed description of the invention proceeds.

It has now been discovered that the new boron phosphides of this invention, particularly. B 1, are extremely satisfactory in the construction and use of point contact rectifying devices.

It has been found that boron phosphides having boronto-phosphorus atomic ratios of at least 6 to l have chemical and physical propenies which render them eminently suitable in semi-conductor devices having, for example, a markedly high thermal stability permitting high operating temperatures. Specifically, these boron phosphides have dissociation pressures of less than 100 microns of Hg at 1200 C. Consequently, these boron phosphidcs are suitably operable at temperatures up to about 2000' C. Compared against the new boron phosphides as a rectifier material are the well known semiconductor materiak germanium, which can only be used at temperatures up to about 80 C., silicon which is operable at 200' C., or the compound 8? which is operable at 1000 C.

Boron phowhides having a boron-phosphorus atomic ratio of at least 6 to 1 exhibit the usual negative temperature coeflicient of resistance of a semiconductor, and have a forbidden energy gap of about 2.0 electron volts as compared with germanium with 0.7 electron volt and silicon with 1.12 electron volts.

The new crystalline rhombohedral boron phosphides of the present invention are extremely hard, thermally stable and chemically inert.

Thcnovelformsofcrystallineboronphosphidedisclosed herein may be prepared by a chemical reaction between elemental horon and elemental by thermal of boron phosphide having the formula 8?, by the reaction of elemental boron with BP, byreaction of elemental boron with the compound phosphme,PH,,orbythereactionofaphosphormsource, suchasferrophosphorusorcrudephosphatemwitha boronsommsuchaselementalboromcmdeboramor other boron compound, in a molten inorganic matrix, such as molten metals or salts thereof.

While the abovealcscribed methods may be used to preparearryofthecrystailineboronphosphideshaving aboron-to-phosphm'usatomicratioofatleast6tol, they are particularly useful for preparing the stoichiometric B P. However, a more preferred methodforpreparinghigherboronphosphidegie, those having boron-to-phosphcrus atomic ratios greater than 6 t01,c0nsisBofheatingtbecompoundB;Pundcrspccific conditions set forth hereinafter. This method '5 preferred because it is susceptible to more accurate means of control for obtaining specific compositions within the above ratio than are the earlier named methods for obtaining the same compositions.

The following specific examples illustrate methods of preparation of the new crystalline boron phosphides under equilibrium conditions:

Example I The transformation of the simple form of boron phosphide having the formula, HP, to the crystalline form having the formula, B 1, was conducted by placing g. of boron phosphide in finely-divided form in a graphite crucible in a porcelain tube located in an electric furnace. Theporcelaintubewasconnectedtoavacuumsystem which could be maintained at 50 microns of Hg pressure. The electric furnace was brought up to a temperature of 1200' C. and maintained constant. It was found that the evolution of phosphorous dtn'ing a 12 period yielded a residual product having the formula. 8 1. It was also found that the starting material could be either amorphous HP or the cubic crystalline form of boron phosphide with the production of the same ultimate product.

The critical comideration required for the transformatiouofBPtoB P,isthatthesystembeoperatedmch thatthepartialpresure()belesthanthat of the decomposition pressure of BP at the ambient tempcrature.

Inthepresentexampletheoperatingpressmeofthe furnace and the temperature at which it is maintained were such that the transformation of HP to B 1 was aided by permitting the evolved phosphorous resulting from the dissociationtoberemovcdfromthereactionzone (by means of the vacuum collection system). The temperature in the phosphorous collection zone was atarelativelylowvaluebytheuse ofawatercondenser, e.g.,pressureofthephosphoroussotn'cewhichwasles lower than the pressure (phoqwhorous) over the product, thus allowing the desired reaction to proceed.

After the processing described above, the product having the-formula, B P, was found tobe a may powder of unusual hardnes. It was found that the gray powder washarderthansiliconcarbidqandhadahardnesou the Mob scale between 9.0 and 9.7.

X-ray analys'u also detected the existence ofauniquecrystallinestructnreforthcabove sample dilfcreut from that of the compound BP.

ExampleZ 'lhereactionofelementalbmonwithelementalphosphorous for the production of hexaboron phosphide,

B 1 was carried out by charging 0.4176 g. of

boronintoagraphitecrucrblewhichhadbeenprepued by drilling-a i "holeinacylindricalpieceof%" graphiterod. 'lhechargedcmciblewasplaoedintoa outside diameterceramictnbc 10"long, dosedat theendnearestthcsample. One-halfofthiscerarmc filledtheentiretube atthedesiredpressure. Thephosphorus vapor then reacted with the hot boron contained at the other end of the tube. It was found that at the end of a heating period of about 24 hours, the boron had been transformed substantially completely to the compound hexaboron phosphide. A similar experiment conducted at 1200' C. was also fotmd to give the substantially quantitative yield of hexaboron pbosphide. In general, the operating pressure which yields the hexaboron phosphide instead of boron phosphide rs in the range of 100 to 1500 microns ofl-Ig at temperatures between 1000' C. and 1200' C. Thus, as shown in FIG. 1, at 1000 C., the pressure of 100 microns of Hg gives hexaboron phosphide while a pressure of 1570 microns of Hg gives boron p p Inthepresentexampletheuseoitheshaped charge of starting material, i.e., boron located in the cavity in the graphite crucible resulted in the production of a amilar and identically shaped product of hexaboron phosphide. This shaped article was found to be stable at high temperatures.

The hexaboron phosphide product was found to have a bulk density of 2.45. However, the ultimate density of individual homogenous particles varies between 2.60 and 2.72. In contrast, cubic crystalline boron phosphide has a theoretical X-ray density of 2.97.

Inthisexample,asintheprecediugexample,thecondition of the formation of hexaboron phosphide is that tlmsystembeoperatedsuchthatthepartialpressme (phosphorus) islessthanthatofthedecompositionpm sure of boron phosphide at the ambient temperature.

The higher boron i.e., those having boronphosphormatomicratiosgreaterthan6to l,arealsopreparedinaccordancewiththismethod,forexarnple;by adding to 67.65 gms. of elemental boron suflicrent elemental phosphorus to form the desired higher boron phide. Forexample,9.6gms.ofreactedwrth 67.65 gms. of boron produces B 1, 4.84 mm. of pins phorus moduces B 1, 2.76 grm. produces B? and 1.94 gins, of produces B P.

Example3 'IheproductionofthecompoundB Pfiomboronphosphidereactedwithexceselementalboromwascarned outataseriesoftemperamres above 1000' C. phomidewasemployedasafinely-dividedcrystallme powda,whiletheboronwasalsoinafinely-dividedform oflessthanloomeshparticlesim.

Thetwoweremixedandchargcdt graphitecrmblehavinganinternalandexternalelement whichfittedloowytogetherwiththespacebetweenthe twoportionsforminganoseconesuchaswas for seinarocket. Theimimatdymixedcombmatlon ofbotonphosphideandelementalboronfimolesof baonpu-moleofboron )washeatedtoa temperattneofl300'Qforaperiodofl8hoursinan inertgas. Attheendofth'stimethecharged cruciblewascooledandthetestpieceremoved. Itwas foundthatthebormphosphidehadbeentransfouned ycompletelytoa'ystallineB Ppmductwh dl wasveryhardandwhichcouldbesubiectedtoondmng orredneingflameswithontsubaantialde 'Ihissamemocedureisfdlowedtoobtainthehighcr boron by the with the phosphorus was directed into a heated reaction vessel, into which gaseous boron trichloride was also flowing. At temperatures of 1100' C. the reaction between the RC1; and the phosphorus result in the formation of the crystalline product B P. However, it is essential that the conditions be such that the partial pressure (phosphorus) be 1m than that of the decomposition presure of boron phosphide at the ambient temperature.

'lhissameproeedm'eisfollowedtoobtainthehigher boron phosphides by increasing proportionately the amount of boron trichloride required to supply free boron suflicient to react with elemental phosphorus to obtain the desired boron-phosphorus ratio.

Example 5 The production of B by the reaction of elemental boronastheboronsourceinsolid formwithphosphine, PH;.asthephosphorussourcesuppliedinagasformwas conducted in aceramic tube located in anelectric furnace. A 10 g. sample of elemental boron held in the furnace for a period of 12 hours with the continuous passage of phosphine over the boron was fotmd to result in a substantially complete transformation to B 1. The neceaary condition for the reaction was that the partial prmne (phosphorus) be less than that of the decomposition pressure of boron phosphide at the ambient temperature.

The higher boron phosphides are similarly prepared by adjusting upwardly the proportion of elemental boron to phoqhine required to produce the desired higher boronphosphons ratio.

Thismethodwasalsofoundtoyieldthedesiredboron phosphide by the reaction of the said elemental solid form of boron with elemental phosphorus carried in an inert gas flame, preferably hydrogen, although argon or nitrogen can also be used.

Example 6 The formation of hexaboron phosphide, BA, in an inorganicmeltwascarriedoutbythereactionofcrystallineBPwithalOmolarexcessofelementalboron. This reaetionwascarrbdoutinaferro-meltbyfirstforming BPfromferro-boronandferrophosphorus. Thisresulted intheproductionofafiuelydispersedformofBPinthe moltenironmatrix. Thelomolarexcessotelemental boronwasthenstirredintothemoltenreadionmedium. 'lhiswasmaintainedatatempemtmeoflwo'cfor aperiodof24hours. Attheendofthistimetheteaction masswaswoledafterwhichtheimnoontentwasm movedbysolutioninsulfuricacid. TheresiduaLinsolubleslndgewasflicnwashedtreatedwiflrhydroflnoricacidandtheu'ystallineformofB Preooveredas thenltimateproduct. 'lhecriterionfortheformationof Misthatthesystembeoperatedsuchthatfliepartial pressure(phosphorus)belessthanthatofthedecompositionpresmeofboronphosphideatambienttemperannes greaterthnnfioo'cthroughouttheeutireptocem.

FIGJshowstheequilibrinmprocessoperatingregion whichhasbemfonndtoyieldthepteferredpmductlkl Thisistherangeof presurebelowthe PQR(areaPQRSlwhereRandQareintereeplsondie 1,000,000 micron of Hg presnre line) and preferably bdowthelineXY(areaVWXY). Thelowerlimitof thisoperatingrangeisthepresmeonemiconofflg. 'lhebroaderoperatingtempetatm-erangehasshownm PIG.1,from800'C.nol947'C.,thepreferredrange beingfmml000C.tol600C. 'Ihepresureraugeis fromltoonemillionmicronsofligtheprefenedrang: beingrepresentedbythelineXY.

Whiletheaboveexampleedescn'betheof thedes'redcrystallinebomnitwillbenoted thatFIG.lrepresentsequilibr-iumforproducing B P. Highcrcrystallineboronie thosehaving atomieratiosgreater than6tolmayalsobepreparednndersimilarnonequilibrium conditions at temperatures between 800 C.

and 2100' C. and pressure of 1 micron of Hg to 100 atmospheres. However, a more preferred method for obtaining the higher boron phosphides consists in heating the compound hexaboron phcsphide 3 under specific conditions set forth hereinafter. This method is preferred because it is susceptible of more accurate means of control for obtaining specific higher boron phosphide compositions than are the earlier discussed methods.

The preferred procedure for obtaining higher boron .phosphides is based upon the fact that when hexaboron phoqhide is heated within a temperature range of from 800' C. to 2100' C. and within a premure range of from lmicronofHgto looatmospheresitundergoesaprw,

'lheinstantboroneg.,B Pcanbedoped withvariousmaterialstoproducetlmdesiredn-orp- D pingisknowninthisartas addingsmallamountsofforeignmaterialstochangethe degreeand/ortypeofamaterial. The treatiitgordopin'gagenttreatmentusedisamethodof g the degree of electronic (or positive hole) inB P. Thedegreeofvarieswith theamountandtypeofdopingagmtused. Forexample, ifitisdesiredduringtheprocesofprodncingmby anyoftheabovemethodsavolatilehalideofaGroup 'H'Belement,i.c.,a'nc,cadmiumormercury,magnesimn aberyllinmcanbexldedtotheractantsinminor togivep-typeB P. Ifann-typelklisdesired theGroupVLBelemmti.e oxygm,sulfnr,seJenium. tellnrinmorpalminmmnbeaddeddnringtheproees higherbmonphosphideisheatedtoatemperatureof Candmbjectedtoatraceamountofthe vaporizeddopingelunmtwhiehisallowedtodimiseinto higher boron phowhide crystal. Normally, long puiodsoftimewillberequiredforthistypeofdoping Whenitisdetermined mm. l m .m h t n yq s rhetoroomtempetature. Thisofcwrseis theandqumchmethodused-for dqingnutaialsafterthecrystallinematerialhmbemmade. lftlnmaterial'iseooledslowly,

me s m fthelatficeagain. Qnenching'traps tho fl gcn withinthecryml 'l'heexampleillustntesaspecificembodiment 6 of point contact rectification experiments carried out using a disc of hesaboron phosphide, B P.

Example '7 A disc of hexaboron phosphide, 13.1 was hot pressed to dimensions of 0.19" x 0.12" x 0.33". was coated on the bottom side with silver paint (suitably other noble metal paint can be used) to provide good ohmic contact with the disc for attachment of an electrode. The disc was placed on top of the gold-plated nickel plate, providing good ohmic contact between the silver-coated side of the disc and the gold-plated nickel plate. The

As an alternative, but les of the present invention. the same procedure recited in Example 7 maybe followed. wherein the surfaeeofsilverpaintisomittedandtheelectrodeisfttsed. solderedorweldeddh'ectlytodiscl I WhenExample7isrqaeatedttsingotherhigherboron p of this invention, comparable rectification ratios and back-voltage resistances are obtained.-

Theinventionwillbemoreclearlynnderstoodfi'omthe followingdetaileddauiptionoftheinventionmadewith .4insuchmannerastothe W1 y arcavailable. uppersurfaceofbody2'sandsprhigwim im' d isuitable mhm-u uam stenwh'skenispresedagimttheuppersm'faeefofd'nc 2mm rectifym'geontacttherewrth. ma m M a- MhomJhkiqmecanvarymm aboutlotoaboutlmgmoremlea fcperformance. lhenpperIendofpoint'cmtactwireSis welded.solderedorotherwiseintegally f electrode which suitably ismckeL' Electroh s encasedinimulatedwhidlsuitably'sceramic,quartzorthelike. Glasscapmlelis totheinsulatedlead-thmughandmetalbmeeycmz ments..snchantasthis allowsthenmint'enanee v vItisvuy dcsiredatmosphueinsidemdsenlrfltlnm leadsl'andllaretoelectrodesliamlrespectivelym'ndtoanalan'rmtsmrcelfito bc rectifiedandanelectricalloadll fllecirect acrmsres'stnrll. 'Snitably,cun entsorncel3canbeall0volt,60 cyelesom'ceofotheraltematingcunentsourceothigher orlowervoltage.

trodethephosphor FIG. 3 is a graph of the data obtained, being a plot of the voltages in volts against the current flowing in the rectifier in milliamperes and indicates a rectification ratio of about 100 to l and a back voltage of about 20 volts without breakdown.

The thermal stability of the group of boron phosphides having boron-to-phosphorus ratios of at least '6 to l is a characteristic property which readily this boron-rich group from the elementary boron phosphide, BP. At temperatures above 1000 C., BP evolves phosphoms copiously resulting in a deleterious atmosphere of phosphorus on and around the components of the rectificr device which is corrosively destructive of the operation thereof. coincidental with this evolution of phosphorous s a breakdown of the physical structure of the BPcomponentdnetoamllapseofitscubicci-ystalline Qrnctnrer On the other hand. the boron phosphides de- Sctibedhereinarenotcubiccrystallineinfmhenoc, even when phosphorus is lost, at much higher tempaa tnresthan 1000' C., thereisnophysical breakdownof the rectifier'component. And, since thephosphorus contentoftheinstantboronphosphidesismuchlowerthan in 8?, there is less phosphorus to evolve into a deleterious thereof around the componens of the rectiflex-device. Asaoonsequencqthebomnphoqhidesde su1'bcdandclai1nedhcreinarefarsuperiortoBP,heing operable at higher temperatures for longer periods of timewithleadangerofoorrosionoftherectifiercompomnts.

Asanillustrationofthecompamtivetimmalstability oftheBPandB P (represeutativeofthenew clasofbmmphosphides),whenBPisheatedtoatemperatureof 1100 C. at loomia'onsofflgpresstneit immediately begins to evolve phosphorus and to decomposeuntilafteraboutwhourstheBPistransfor-med C. and 100 micron of ofvariouselementsandbormphosphide compoumisandinappllcations. Itisapparmttlntthemwcrystaflineboronphosphidcsofthepresntinventiomhavingabmun-tophosphormawmicratioofatleast6tolareoperahleattemperaturesfarprimartmatuials.

Itwillbeapparmttothoseskilledinthearttlntmitablecquivalentmataialsmaybesubstitntedforthevarh ousmboronphosphitbintherectifierof thisinvention. Forexamplewhueasinthespecificemdma'ibedhuehtsilvu'paintwxusedtomakc olmiceontactbctwcmthebcl'ond'mcand clcctrodciodlersnitablemctalscanbeusctalsmodrcr mta'ialsthangold-platednickelcanbeuscdas-theelecnodeincontactwith-thesilverpaintorthe'nickelelecmortimgtenwireto theuppcrsideofthebormphtsphidedisc. Thespring wirepointcontactelectmdeitselfmaybeanodrersuitable mtet'nl. Thesa meappliestotheglampsuleand ceramic Itwillbemflcrstoodbythoseskilledintheartthat thefdcscriptimroftheinternnof'a specifiedishywayofonlyandthat theinventim'snotnecesarflylimitedtherctoinvicw oicqfivalemallernativeemapparenttothose skilledinthe art. ,areconwhiehcanhemadewithoutdepartinghmnflte spifitandscopeoflhetksuibedinvmtion.

Othametakwhidtaredfectivetoprovideohmie contacts-with the boron phosphides are platinum and its alloys, such as platinum-rhodium.

What is claimed is:

1. A point contact rectifier device comprising]: boron phosphide body having a boron-to-phosphorns atomic ratiowithintherangeoffmm6tolto 100th 1,anelectrodemakingohmiccontacttherewithandapointcontact electrode in. contact with said body.

2. Apoint contaetrecfifierdevice comprising'a boron 'phosphide body having a boron-toatomic ratio within therange offrom 6 to 1 noon, 1, am\ ducting element on a portion of said body'to facilitate clectriml connection thereto, an

contactwithsaid body through said element amiapointcontactelectrodeincontactwith'saidbody.

3. Deviceofciaitnlwhereinmidbodyisintheform otadisc.

4. Device otclaiml whereinthepointcontacteiectrodeisalhosplmrlxonzewire.

5.Devieeoclaimlwhcreinthepointcontactelee-,

atomicratiowithinthenngoifiomStol to 100m 1, mesidewitha conductingsurfmafizstmctaldedmdeattadqedto theconductingsurfaceotsaiddiseand tojasccondmetalcicdmdewhichismcasedhzanon- -tln'ough,ampsulecmlosingsaiddiscand tosaidfirstmctalelectrode.

l3.'l hedevioeoiclaim,1 2whereinthefirstmetal clectrodeisnoble u thesurfaceis anoblemetalcondncfingsurfaceanrl flnpointcmct eledmdeisblme.

a 14. beviceofclaimlzwhereinsaiddiscisthebmm IiDeviceofclaimlZwhereinsaiddiscistheboron lQDeviceofclaimlZwhcminsaidrfisckflnbmm Mile-B 17.D eviceoclaiml2whereinsaiddisc'nthebmm l8.Deviceoclaiml2wheremsaiddhckthe Blar- Menneafitedinthefikofthispatent UNITE) sums m'rmrs 2,908,851' Milieaetal. 4 -oa.1s,19s9

2,939,058 Mastcrsm May 31, 1960 2,980,833 Epstein All. 18, 196! FOREIGN PATENTS 719,873 Great Dec. 8, 1954 deviceeompisingaboron 

1. A POINT CONTACT RECTIFIER DEVICE COMPRISING A BORON PHOSPHIDE BODY HAVING A BORON-TO-PHOSPHORUS ATOMIC RATIO WITHIN THE RANGE OF FROM 6 TO 1 TO 100 TO 1, AN ELEC- 